The invention is related to image sensors, and more particularly, to reducing pixel reset noise in image sensors.
In image sensor applications, it is becoming increasingly important to integrate the image sensor with other circuitry for driving the image sensor and for performing on-chip signal processing. Ideally, the integration of on-chip circuitry with image sensors should provide good imager performance with low-noise, no lag or smear, good blooming control, random access, simple clocks, and fast readout rates.
To meet these requirements, image sensors compatible with on-chip circuit technology, such as Complementary Metal-oxide Semiconductor (CMOS) technology, have been developed. One previously demonstrated CMOS compatible technology is active pixel sensor (APS) technology, which includes: amplified MOS image (AMI), charge modulation device (CMD), bulk charge modulated device (BCMD), and base stored image sensor (BASIS). Although AMI is compatible with CMOS, the device suffers from high noise and lag problems making it unsuitable for high resolution applications. CMD, BCMD, and BASIS can be made compatible with CMOS technology, but these devices typically require additional fabrication steps thereby making it difficult for such devices to successfully compete against charge-coupled-devices (CCDs); the dominant technology for image sensors. See, e.g., S. Mendis et al., xe2x80x9cProgress in CMOS Active Pixel Image Sensors,xe2x80x9d in Proceedings of SPIE, pp. 1-2, (San Jose, Calif.), February 1994.
Noise in CMOS image sensors is typically much larger than noise in CCDs. The noise can be categorized as either fixed pattern or temporal. Fixed pattern noise can be eliminated by using pixel-to-pixel offset and gain correction. On the other hand, temporal noise typically cannot be removed after it is added to image data. Thus, techniques for reducing temporal noise in CMOS image sensors must be developed.
Temporal noise in standard APS is well understood. The largest temporal noise component is contributed by resetting the pixel, and is on the order of       kT    c    .
In a CCD, temporal reset noise can be eliminated by using correlated double sampling (CDS), as described in T. Nobusada et al., xe2x80x9cFrame Interline CCD Sensor for HDTV Camera,xe2x80x9d in ISSCC Digest of Technical Papers (San Francisco, Calif., USA), February 1989. Unfortunately, due to size limitations, pixel level CDS cannot be used in APS.
More recently, a technique was developed that reduces temporal reset noise to   kT      2    ⁢    c  
without the addition of lag. See B. Pain et al., xe2x80x9cAnalysis and Enhancement of Low-light-level Performance of Photodiode-type CMOS Active Pixel Images Operated With Sub-threshold Reset,xe2x80x9d in 1999 IEEE Workshop on CCDs and AIS, (Nagano, Japan), June 1999. Although this technique significantly reduces temporal reset noise, more noise reduction is needed for CMOS image sensors to successfully compete with CCDs in the market.
Accordingly, there is a need for a pixel reset circuit and method for reducing pixel reset noise in image sensors to improve image clarity. Such a circuit and method should reduce pixel reset noise without adding lag, and be directly applicable to existing APS designs.
The present invention is directed to reducing pixel reset noise in image sensors without adding lag. An image sensor (e.g., APS) having active reset readout includes a circuit for resetting a pixel device. The reset circuit includes a switch coupled to a readout node for resetting the pixel device. A compare module is coupled to the switch, the readout node, and a reset signal for turning the switch on and off in response to a difference detected between the reset signal and a readout signal from the readout node. The capacitance in the circuit is coupled to reduce noise in the readout signal.
A method of the present invention includes providing low noise readout from a pixel device in an image sensor. The image sensor includes a reset circuit having a reset control loop for controlling the reset of the pixel device. The reset control loop preferably includes a compare module and a switch. The compare module has a first input for receiving a reset signal and a second input for receiving a feedback signal from a readout node. The readout node is coupled to outputs of the switch and pixel device, respectively. The method comprises the steps of: (a) comparing the reset signal with the feedback signal; (b) turning the switch on and off in response to a difference detected between the reset signal and the feedback signal; and (c) controlling the reset control loop with capacitance in the reset control loop in response to the switch turning off, wherein the feedback signal reduces noise at the readout node.
In the preferred embodiment of the present invention, the reset control loop includes a reset amplifier and a transistor. The reset amplifier has a first input coupled to a reset voltage and a second input coupled to a readout node (e.g., the xe2x80x9csourcexe2x80x9d of the transistor). When the reset voltage exceeds the readout node voltage, the reset amplifier output voltage rises and turns on the transistor. After turning on the transistor, the output voltage of the transistor follows the reset voltage until the reset voltage stops rising and the readout node voltage overshoots the reset voltage. After the readout node voltage overshoots the reset voltage, the reset amplifier output voltage drops and turns the transistor off. With the transistor off, only overlap capacitance of the transistor is used to control the reset control loop. The overlap capacitance is combined with pixel capacitance to form a capacative voltage divider in the reset control loop for reducing pixel reset noise at the readout node. When the reset voltage is returned to ground or other suitable reference, the pixel reset is complete.
An advantage of the present invention is the use of bandlimiting and capacitive feedback to reduce pixel reset noise in image sensors without adding lag. By reducing pixel reset noise, the fundamental detection limit of the pixel is increased resulting in improved image clarity. The present invention can be applied directly to existing APS designs with minimal design changes.